Many portable, battery-powered devices (e.g. mobile handsets, keyboards, personal digital assistants) recharge their batteries through a DCP (dedicated charging port) such as an AC-to-DC (alternating current to direct current) wall-wart adapter or a hub port. A wall-wart is small power supply brick with an integral male plug. The wall-wart is designed to plug directly into a wall outlet. It is called a “wart” because when it is installed on a power strip it tends to block at least one more socket than it uses.
These portable, battery-powered devices may also recharge their batteries through a CDP (charging down-stream port). A CDP usually has a five volt power supply provided from the USB (universal serial bus) ports on PCs (personal computers) such as laptop, desktop and notebook PCs. Commonly, the portable device has only one input which is designed for USB compatibility in order to save space (as opposed to having a separate wall-wart connector).
When a portable device has one input, for example a USB compatible input, the portable device usually cannot differentiate when it is plugged into a CDP versus being plugged into a DCP such as a wall-wart adapter. Usually, a CDP has a current limit of 500 ma (milliamps) while a DCP can provide much more current. For example, some dedicated charging ports can provide 1.5 amps to a portable device. Because most portable devices cannot differentiate when they are plugged into a CDP versus being plugged into a DCP, most portable devices limit the amount of current they draw to ensure that the portable devices do not overload the power source.
Several recent standards have defined protocols that allow a portable device to differentiate between different power sources to allow the portable device to draw maximum current from a source. Drawing maximum power from the source reduces the time required to charge the portable device. Several recently released industry standards such as the Chinese PRC Telecommunications Industry Standard YD/T 1591-2006 and USB 2.0 Battery Charging Specification 1.1 (BSC1.1) define both power sources, to handshaking protocols and allowable current draws. Other proprietary-based schemes (such as Apple's method for charging iPods and iPhones) are company specific. These standards define how a portable device communicates with a host device to determine the maximum allowable current draw.
These different standards are not necessarily compatible with each other. For example, a portable device following one standard may not communicate with a host device following a second standard.